The central hypothesis of this application is that thymic involution results from age-related changes in bone marrow T cell precursors, intrathymic progenitors, and thymic stromal cells. One aim is to define and characterize effects intrinsic to each of these populations. A second goal is to determine how concomitant declines in the production of Growth Hormone (GH) and Insulin-Like Growth Factor-I (IGF-I), which are thymopoietic, and increases in production of sex steroids, which potentiate thymocyte death, at puberty impact on cells in the bone marrow and thymus. Studies in Aim 1 will investigate why pluripotent hematopoietic stem cells (PHSC) from old mice do not efficiently generate T cells by testing the hypothesis that their differentiation into common lymphoid progenitors (CLP) and/or more committed T cell progenitors is compromised by aging. In addition, whether or not age-related changes in the production of GH, IGF-I or sex steroids affect the size of the PHSC and CLP compartments will be determined. Aim 2 will investigate how aging affects the pool of intrathymic progenitors. The most immature CD44+CD25- precursors in the thymus are present at a normal frequency but their maturation is blocked. Studies will determine whether this is due to intrinsic changes that accumulate in this population and/or to alterations in the balance of microenvironmental and endocrine signals that affect their growth and survival. Finally, Aim 3 will assess if and how aging affects thymic stromal cells using a newly developed, adult thymus reaggregate culture system. In addition, fetal thymic organ cultures will be used to model how age-related changes in hormone production noted above impact on the ability of thymic stromal cells to support T cell development. Taken together, the information obtained from these studies will define the basis for thymic involution and be of value in the formulation of strategies to boost cell-mediated immunity and rejuvenate the thymus in immunocompromised individuals.